Bookmarks are sorted by folder or categories in browsers by default. However, with downloadable extensions, you can browse your bookmarks by date of bookmarking.
DSBMarks is a Firefox addon that creates bookmarks within folders organized by date.
You have to use the specific icon at the bottom right of Firefox to create bookmarks within sorted folders. It leaves previous bookmarks intact, so you can bookmark with and without the date-based hierarchical ordering. I personally would have preferred an option to view/sort existing bookmarks too.
To view your bookmarks, go to the Bookmark and scroll down to year folder, then month and date. For instance, a bookmark on 6 August 2010 will be in 2010 > 08 > 6.
Get DSBMarks addon [addons.mozilla.org]